1. Field of the Invention
The present invention relates to a technology for controlling power output from a power generating device, and more particularly to a control technology for plural interconnected power generating devices.
2. Description of the Related Art
Recently, technologies for realizing use of energy harvesting power generating devices in various types of systems are under development. The “power generating devices” is a general term referring to devices which generate electric energy from other energies such as kinetic energy, thermal energy, light energy, electromagnetic energy, and chemical energy, including turbine generators, thermoelectric conversion elements, solar cells, fuel cells, oscillation power generating elements, and electromagnetic power generating elements. The “energy harvesting” refers to a technology which collects natural energies such as solar energy, wind power energy, hydraulic power energy, and geothermal energy, or energies such as oscillation energy, thermal energy, light energy, and electromagnetic energy released from a certain system to the surrounding environment, and reuses these energies collected as power. For example, an image forming apparatus disclosed in JP 2013-025280 A collects waste heat released from a fixing unit by using thermoelectric conversion elements, and reuses the collected power for driving of an exhaust fan or for temperature retention or heating of the fixing unit. In addition, there is known a portable wireless communicator which collects oscillation given to the communicator by using oscillation power generating elements, and reuses the collected power as auxiliary power supply for avoiding battery exhaustion, for example.
Maximum power point tracking (MPPT) control is known as a technology for maintaining high power generation efficiency of a power generating device. The “maximum power point” corresponds to a peak which appears in power-voltage characteristics of a general power generating device. When output voltage of a power generating device is equivalent to voltage at the maximum power point, power output from the power generating device becomes the maximum. The maximum power point considerably varies in accordance with fluctuations of the environmental conditions. For example, maximum power points of thermoelectric conversion element and fuel cells are dependent on temperatures, while maximum power points of solar cells are dependent on volumes of sunshine duration. According to MPPT control, variations of the maximum power point are calculated from measurements of the environmental conditions such as temperatures and volumes of sunshine duration, or from fluctuations of output from a power generating device. Then, the output of the power generating device is adjusted to follow the calculated variations of the maximum power point. According to MPPT control disclosed in JP 2008-138636 A, for example, the maximum power point is calculated from measurements of wind velocity by using a relational expression between wind velocity and maximum power point. Then, output voltage of a wind turbine generator is adjusted to voltage corresponding to the calculated maximum power point. According to MPPT control disclosed in JP 2013-101500 A and JP 2007-133765 A, the product of voltage and current of output from a solar cell, i.e., output power is calculated every time voltage and current of output from the solar cell are measured. Then, the output of the solar cell is adjusted such that the current value of the product becomes larger than the product calculated immediately before the current product. In any types of the foregoing MPPT controls, the output of the power generating device is maintained at the maximum power point, wherefore the power generation efficiency is maintained at a high level.
In practical application, a plurality of power generating devices are equipped in a system and interconnected with one another within the system. Particularly, in case of an energy harvesting power generating device, the amount of power generated from a single power generating device is small, or the stability of operation for fluctuations of environmental conditions is insufficient. Accordingly, a system intended to include a power generating device generally carries plural power generating devices, particularly such power generating devices having different operational characteristics for environmental conditions, such as solar cells, radio wave power generation circuits, and thermoelectric conversion elements, in a manner interconnected with one another within the system. According to a control system disclosed in JP 2013-110865 A, for example, oscillation and heat generated from an automobile, and sunlight and electromagnetic waves received by the automobile are collected as power by using various types of power generating devices mounted on the single automobile. This structure supplies a sufficient amount of power from the whole power generating devices, and maintains this amount of power with sufficient stability regardless of fluctuations of the environmental conditions.
The followings are known as technologies capable of improving power generation efficiency of a system including plural interconnected power generating devices, for example. The system disclosed in JP 2008-138636 A individually performs MPPT control for each of plural wind turbine generators, and extracts power from a power generator having the highest voltage at the maximum power point. The system disclosed in JP 2013-101500 A individually performs MPPT control for each of strings during interconnected operation between plural solar cell strings and external power systems, and adjusts output voltages of all the strings to a uniform voltage during independent operation of the strings isolated from the external power systems. The uniform output voltage is adjusted such that the power consumption by loads can be covered by the total sum of the power outputs from the respective strings, wherefore concentration of the burden only on any one of the strings is avoidable. The system disclosed in JP 2007-133765 A individually measures output characteristics of solar cell modules corresponding to interconnection targets. When voltage of any one of the modules at the maximum power point does not reach a lower limit of input voltage of an inverter disposed downstream, modules in similar situations and having the same current at the maximum power point are connected in series. Then, the synthesized output is input to the inverter. In addition to the foregoing technologies, the followings are considered as easy control technologies to be put into practice: (1) selecting a power generating device which generates the largest amount of power at the maximum power point, and supplying only the output from the corresponding power generating device to loads; and (2) adjusting output voltages of all power generating devices to voltage of a power generating device which generates the largest amount of power at the maximum power point.
In any cases of the foregoing technologies, it is obvious that the total sum of the amounts of power extracted from the whole power generating devices is smaller than the amount of power generated from the whole power generating devices. For example, the system disclosed in JP 2008-138636 A does not extract power from the power generating devices other than the power generating device having the highest voltage at the maximum power point. According to the system disclosed in JP 2013-101500 A, the uniform output voltage at the time of independent operation is generally different from each voltage of the strings at the maximum power point. As apparent, any of the foregoing technologies can extract not the entire amount of power generated from the respective power generating devices, but only a part of the amount of the power. Accordingly, the power generation efficiency is expected to further improve when the amount of power extracted from the whole power generating devices increases. However, no specific method for increasing the amount of extracted power is known, nor is obvious even for those skilled in the art.
For realizing extraction of the maximum amount of power allowed to be generated from all power generating devices, the following method may be adopted only for achieving this purpose. MPPT control is individually performed for each of the power generating devices to extract power at voltages corresponding to the respective maximum power points from the power generating devices. Then, the respective voltages are adjusted to a common voltage by using constant voltage circuits. According to this method, constant voltage circuits as many as the power generating devices are needed, wherefore it is difficult to further decrease the scale of the whole system which controls the power of the respective power generating devices. This situation is undesirable in consideration that further reduction of the manufacturing cost of the system becomes difficult.